Echinoderms 459

CHAPTER 23 Echinoderms 459 Position in Animal oping elsewhere; coelom budded off unique constellation of characteristics Kingdom from the archenteron (enterocoel); found in no other phylum. Among the radial and regulative (indeterminate) more striking features shown by 1. Phylum Echinodermata (e-ki no- cleavage; and endomesoderm (meso- echinoderms are as follows: der ma-ta) (Gr. echinos, sea urchin, derm derived from or with the endo- a. A system of channels composing hedgehog, derma, skin, ata, derm) from enterocoelic pouches. the water-vascular system, characterized by) belongs to the 3. Thus echinoderms, chordates, and derived from a coelomic compart- Deuterostomia branch of the animal hemichordates are presumably ment. kingdom, the members of which are derived from a common ancestor. b. A dermal endoskeleton com- enterocoelous coelomates. The other Nevertheless, their evolutionary his- posed of calcareous ossicles. phyla traditionally assigned to this tory has taken the echinoderms to c. A hemal system, whose function group are Chaetognatha, Hemichor- the point where they are very much remains mysterious, also enclosed data, and Chordata. unlike any other animal group. in a coelomic compartment. 2. Primitively, deuterostomes have the d. Their metamorphosis, which following embryological features in changes a bilateral larva to a radial common: anus developing from or Biological Contributions adult. near the blastopore, and mouth devel- 1. There is one word that best describes echinoderms: strange. They have a environment, while radiality is of value known, but a few are commensals. On Echinoderms to animals whose environment meets the other hand, a wide variety of other Echinoderms are marine forms and them on all sides equally. Hence, the animals make their homes in or on include sea stars (also called star- body plan of present-day echinoderms echinoderms, including parasitic or fishes), brittle stars, sea urchins, sea seems to have been derived from one commensal algae, protozoa, cteno- cucumbers, and sea lilies. They repre- that was attached to the bottom by a phores, turbellarians, cirripedes, cope- sent a bizarre group sharply distin- stalk, had radial symmetry and radiat- pods, decapods, snails, clams, poly- guished from all other animals. Their ing grooves (ambulacra) for food gath- chaetes, fish, and other echinoderms. name is derived from their external ering, and had an upward-facing oral Asteroids, or sea stars (Figure 23-1), spines or protuberances. A calcareous side. Attached forms were once plenti- are commonly found on hard, rocky endoskeleton is found in all members ful, but only about 80 species, all in surfaces, but numerous species are at of the phylum, either in the form of class Crinoidea, still survive. Oddly, home on sandy or soft bottoms. Some plates or represented by scattered tiny conditions have favored survival of species are particle feeders, but many ossicles. their free-moving descendants, al- are predators, feeding particularly on The most noticeable characteristics though they are still quite radial, and sedentary or sessile prey, since sea of echinoderms are (1) spiny endo- among them are some of the most stars themselves are relatively slow skeleton of plates, (2) water-vascular abundant marine animals. Neverthe- moving. system, (3) pedicellariae, (4) dermal less, in the exception that proves the Ophiuroids—brittle stars, or ser- branchiae, and (5) radial or bira- rule (that bilaterality is adaptive for pent stars (see Figure 23-11)—are by dial symmetry. No other group with free-moving animals), at least three far the most active echinoderms, mov- such complex organ systems has radial groups of echinoderms (sea cucum- ing by their arms rather than by tube symmetry. bers and two groups of sea urchins) feet. A few species are reported to Echinoderms are an ancient group have evolved back toward bilaterality. have swimming ability, and some bur- of animals extending back to the Cam- Echinoderms have no ability to row. They may be scavengers, brow- brian period. Despite the excellent fos- osmoregulate and thus rarely venture sers, or deposit or filter feeders. Some sil record, the origin and early evolu- into brackish waters. They occur in all are commensal in large sponges, in tion of echinoderms are still obscure. It oceans of the world and at all depths, whose water canals they may live in seems clear that they descend from from intertidal to abyssal regions. great numbers. bilateral ancestors because their larvae Often the most common animals in the Holothurians, or sea cucumbers are bilateral but become radially sym- deep ocean are echinoderms. The (see Figure 23-21), are widely preva- metrical later in development. Many most abundant species found in the lent in all seas. Many are found on zoologists believe that early echino- Philippine Trench (10,540 m) was a sea sandy or mucky bottoms, where they derms were sessile and evolved radial- cucumber. Echinoderms are virtually lie concealed. Compared with other ity as an adaptation to sessile exis- all bottom dwellers, although there are echinoderms, holothurians are greatly tence. Bilaterality is of adaptive value a few pelagic species. extended in the oral-aboral axis. They to animals that travel through their No parasitic echinoderms are are oriented with that axis more or less v | v | e-Text Main Menu | Textbook Table of Contents | 460 PART 3 The Diversity of Animal Life Characteristics of Phylum 5. A unique water-vascular system of rounded by extensions of coelom Echinodermata coelomic origin that extends from (perihemal sinuses); main circula- the body surface as a series of tion of body fluids (coelomic fluids) 1. Body unsegmented (nonmetameric) tentacle-like projections (podia, or by peritoneal cilia with radial, pentamerous symme- tube feet) that are protracted by 10. Respiration by dermal branchiae, try; body rounded, cylindrical, or increase of fluid pressure within tube feet, respiratory tree star shaped, with five or more radiat- them; an opening to the exterior (holothuroids), and bursae (ophi- (madre-porite or hydropore) ing areas, or ambulacra, alternating uroids) with interambulacral areas usually present 11. Excretory organs absent 2. No head or brain; few specialized 6. Locomotion by tube feet, which pro- 12. Sexes separate (except a few her- sensory organs; sensory system of ject from the ambulacral areas, by maphroditic) with large gonads, sin- tactile and chemoreceptors, podia, movement of spines, or by move- gle in holothuroids but multiple in terminal tentacles, photoreceptors, ment of arms, which project from most; simple ducts, with no elabo- and statocysts central disc of body rate copulatory apparatus or sec- 3. Nervous system with circumoral ring 7. Digestive system usually complete; ondary sexual structures; fertilization axial or coiled; anus absent in ophi- usually external; eggs brooded in and radial nerves; usually two or three systems of networks located at uroids some different levels in the body, varying 8. Coelom extensive, forming the peri- 13. Development through free- in degree of development according visceral cavity and the cavity of the swimming, bilateral, larval stages to group water-vascular system; coelom of (some with direct development); 4. Endoskeleton of dermal calcare- enterocoelous type; coelomic fluid metamorphosis to radial adult or ous ossicles with spines or of cal- with amebocytes subadult form; radial cleavage and careous spicules in dermis; covered 9. Blood-vascular system (hemal sys- regulative development by an epidermis (ciliated in most); tem) much reduced, playing little Autotomy and regeneration of lost 14. pedicellariae (in some) if any role in circulation, and sur- parts conspicuous parallel to the substrate and lying on one side. Most are suspension or deposit feeders. Echinoids, or sea urchins (see Fig- ure 23-16), are adapted for living on the ocean bottom and always keep their oral surface in contact with the substratum. ―Regular‖ sea urchins pre- fer hard bottoms, but sand dollars and heart urchins (―irregular‖ urchins) are A B usually found on sand. Regular urchins, which are radially symmetrical, feed chiefly on algae or detritus, while irregulars, which are secondarily bilateral, feed on small particles. Crinoids (see Figure 23-26) stretch their arms out and up like a flower’s petals and feed on plankton and sus- pended particles. Most living species become detached from their stems as adults, but they nevertheless spend C D most of their time on the substrate, holding on by means of aboral ap- Figure 23-1 pendages called cirri. Some sea stars (class Asteroidea) from the Pacific. A, Cushion star Pteraster tesselatus can secrete incredible quantities of mucus as a defense. B, Choriaster granulatus scavenges dead animals on The zoologist who admires the shallow Pacific reefs. C, Tosia queenslandensis from the Great Barrier Reef browses encrusting fascinating structure and function organisms. D, Crossaster papposus, rose star, feeds on other sea stars. of echinoderms can share with the v | v | e-Text Main Menu | Textbook Table of Contents | CHAPTER 23 Echinoderms 461 layperson an admiration of the beauty damages the delicate epidermal mem- shoreline where large numbers may of their symmetry, often enhanced by brane, destroying the dermal branchiae aggregate on rocks. Sometimes they bright colors. Many species are rather and ultimately the animal itself. Unfor- cling so tenaciously that they are diffi- drab, but others may be orange, red, tunately, other soft-bodied inverte- cult to dislodge without tearing off purple, blue, and often multicolored. brates are also damaged. However, the some tube feet. They also live on Because of the spiny nature of oysters remain with their shells tightly muddy or sandy bottoms and among their structure, echinoderms are not closed until the quicklime is degraded. coral reefs. They are often brightly col- often prey of other animals—except Echinoderms have been widely ored and range in size from a centime- other echinoderms (sea stars). Some used in developmental studies, for ter in greatest diameter to about a fishes have strong teeth and other their gametes are usually abundant and meter across.

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